The physiology of dopamine (DA) neuronal systems has been a major topic of investigation due to the hypothesized involvement of these neurons in various clinical and neurological disorders, such as schizophrenia, Parkinson's disease, and the side effects of chronic antipsychotic drug administration. Although in vivo electrophysiological studies have proven valuable in ascertaining the responses of DA neurons to pharmacological intervention in the whole animal, these studies have proven insufficient in uncovering the ionic mechanisms underlying these actions. By using in vitro intracellular recording techniques, is should be possible to characterize the membrane conductance changes which are responsible for regulating spontaneous activity, as well as the ionic mechanism which underlie the responses of DA neurons to drug administration. In order to derive data specific to this neurochemical system, the dopaminergic nature of the neurons recorded must first be established. The direct identification of DA neurons in the slice will be done by combining intracellular injection of fluorescent dyes with catecholamine histofluorescene. Furthermore, morphological characterization of identified DA neurons will be accomplished by intracellular injection of Lucifer yellow and horseradish peroxidase. Injection of dyes into the know postsynaptic target sites of DA neurons prior to recording and intracellular staining will permit comparisons of DA neuron morphology and electrophysiology based upon their sites of termination. The next stage of the study will involve determination of the biophysical properties of DA neurons to establish the electrophysiological bases against which responses to drugs can be measured. Thus, sodium spikes, low threshold and high threshold calcium spikes, and pacemaker depolarizations will be investigated by pharmacological blockade of specific ionic channels. Furthermore, the threshold and voltage dependencies of both active and passive ion fluxes will be determined in order to establish the presence of voltage activated or inactivated conductances. The direct effects of DA on neuronal activity and the involvement of the DA autoreceptor in the regulation of cell firing will be examined by the administration of DA agonists and antagonists. In summary, these studies are designed to identify the nature of DA cell autoregulation of spontaneous activity, with special emphasis on the differential modes of regulation expressed by DA cell subpopulations. It is hoped that these studies will provide insights into methods by which we may pharmacologically manipulate, in a highly specific manner, the activity level of this cell group to aid in the alleviation of a variety of clinical disorders.